Means and method of producing an X-ray focus varying with the x-ray tube load



A. MUTSCHELLER 1,923,876 MEANS AND METHOD OF PRODUCING AN X-RAY FOCUS VARYING WITH THE X-RAY TUBE LOAD Filed Feb. 25, 1931 INVENTOR 1 7. MUZSCf/EL 777 Aug. 22, 1933.

ATTORNEY Patented Aug. 22, 1933 PAT MEANS AND METHOD OF PRODUCING AN X-EAY FOCUS VARYING WITH THE X-RAY TUBE LOAD Arthur Mutscheller, New York, N. Y., assignor to Wappler Electric Company, Inc., a Corporation "of New York Application February 25, 1931 Serial No. 518,050

6 Claims. (01. 250 35) smaller than the actual focal area and in which both the effective focal spot and actual focal My invention relates to X-ray tubes and has particular relation to an X-ray tube wherein the focal area is varied with the load impressed upon the electrodes of the tube.

In X-ray tubes in which the emission of electrons from a thermionic cathode forms a round focal spot upon the anode the current supplied to the tube is necessarily limited. This limitation is caused by the resulting radiographic pictures produced by the X-rays radiating from the anode of the X-ray tube. does not vary in any manner other than intensity with variations of current any appreciable increase of current results in a damaged focal spot and where the period of exposure is of a few seconds duration a blurred orjover-exposed radiographic picture also results. 9

A further limitation is imposed upon the current and also upon the useful life of the X-ray tube,- by the utilization of a cathode forming a round focal spot, which is due to the decreased heat connection of a circular area. As a round focal spot is concentrated upon the anode target and the heat convection of a circular area is less, because of the smallness of the border line, than the same area of a different configuration the current impressed upon the tube is limited by the effect of the focal spot upon the v anode, which if too intensified, will destroy the anode. This necessarily limits the current during any one period of energization of the tube as well as limits the useful life of the tube. This latter is due to the focal spot being confined to such a small area that eventually the anode is destroyed in a much shorter period of time than if a focal spot of a different configurationand of an increased heat convection were employed.

7 It is, therefore, an object of my invention to remedy the above noted conditions by providing in an X-ray tubea cathode of line, oval, or

' V-shape in which a focal area is produced upon an anode target having increased heat convection characteristics and in which the focal area is varied in accordance with variations in the load imposed upon the X-ray tube. I

Another object of my invention is the provision of a cathode for anv-X-ray tube which'is so constructed and arranged with a cooperating anode that the focal area is simultaneously varied with variations in the load imposed uponthe tube. r I A further object of my invention is the provision of a cathode for an X-ray tube of such a configuration that when cooperating with an anodetarget produces an effective focal spot As the focal spot area are varied in accordance with of load imposed upon the tube.

variations Still further objects of my invention will become apparent to those skilled in the erence to the accompanying drawing art by ref wherein Fig. 1 is an elevational view partly in crossseotion showing my device as incorporated in an X-ray tube;

Fig. 2 is an elevational view of my cathode and parts immediately associated therewith taken on the line II-11 of Fig. 1;

Fig. 3 is an elevational view of a portion ofv the X-ray tube taken on the line IIIIII of Fig. 1; Fig. 4 is an imaginary view of the effective focal spot projected by my device taken on the line IV-lV of Fig. 1;

Fig. 5 is an imaginary View of the eifective focal spot projected by. my device taken on the line V V of Fig. 1; and

Fig. 6 is a partial elevational view of my device showing a reversed position of the cathode as shown in Fig. 1.

partially between the legs of the oath dual purpose of supporting the latter ode for the and focusing the. electrons emanating from the cathode upon the anode 3.

A small hook 6 is embedded in a small insulating section 7, providedin the shield 5, for the purpose of supporting the filamentary cathode at the apex and a similar section 8 passes through the focusing shield and is arranged to support one of the legs of the ii-shaped cathode. The remaining leg of the cathode is rigidly secured to the metallic focusing shield and is therefore in electrical conductive communication therewith. A conductor 9 passes through the enclosing envelope 2 and is rigidly securedto the focusing shield 5, while a similar conductor 10 likewise passes through the envelope andconn'ects with one of the legs of the filament through the;

insulated section 8. I The cathode is thus'heatedgto an ele ctron emitting temperature by means of the conductors 9 and 10 which are connected to a suitable source of electrical energy (not shown).

The metallic focusing shield 5 presents an angular face toward the anode 3 as shown more clearly in Figs. 1 and 6. This shield extends further beyond the filamentary cathode 4 at the base of the legs than it does at the apex and thus produces a greater electrostatic field at the base than at the apex for a purpose to be herein more fully set forth. The anode 3 is likewise provided with an angular face which is at a slightly greater angle than the usual angle of 45 normally employed. In the usual operation of an X-ray tube the X-rays radiating from the anode in alignment with the perpendicular axis of the tube are normally employed and as the anode is disposed at an angle of 45 the effective focal spot is equal to the actual focal area upon the surface of the anode. By increasing the angle of the anode and employing the slant rays radiating in alignment with the perpendicular axis of the tube I produce an effective focal spot which is smaller than the actual focal area of the anode target and a spot is produced upon an object 11, if placed at this position, as shown in Fig. 4.

Should, however, it be desired to produce an effective focal spot more closely approximating the actual focal area of the anode the object 11 would be placed at an angle to the perpendicular axis of the tube which would have the same effeet as increasing the angle of the anode and a focal spot would, therefore, be produced as shown in Fig. 5.

In this latter position of the object the spot is not concentrated and an area is produced corresponding to the spacing between the legs of the V-shaped focal area which causes distortion in the resulting radiographic film or on a fluor'escent screen. It is, of course, to be understood that in actual practice the X-ray tube is provided with a lead glass shield, (not herein shown) which is provided with a window pervious to X- rays that allows only the radiation of the X-rays inthe desired direction to strike an object.

The emission of electrons from the cathode l will be greater at a given amount of current and temperatures from the apex of the V-shaped filament than along the legs of the filament. This is due to the close proximity of the helical windings in this region which tend to heat one an other and thus increase the temperature. The electrostatic field is also less concentrated in this region by the particular angle of the focusing shield which extends farther beyond the cathode at the base of the legs than at this point. The increasein temperature and a weaker field will therefore result in a greater electron emissivity in this region although the emissivity will likewise extend along the legs.

The actual focal area produced upon the anode 3 will therefore be more concentrated at a point corresponding to this greater electron emitting region of the cathode. When an increase of current results due to hum rous causes, such-for instance, as line fluctuations, the temperature of the cathode will likewise increase. As an increase in temperature results in greater electron emissivity the emission will become greater and extend farther along the legs of the V-shaped filamentary cathode.

a This increase in emissivity will naturally increase the actual focal area upon the anode 3 and it necessarily follows that the effective focal spot will likewiseincrease with the increase in the actual focal area. Due to the angle of the anode and the point at which the X-radiation is utilized the effective focal spot varies with va-- riations in current and actual focal area but it never equals the actual focal area.

In Fig. 6 I have shown an inverted form of the V-shaped cathode as shown in Fig. l together with a focusing shield and a cooperating anode. The results obtained from this configuration are identical with the one just described the only difference being in the inverted form of the projected actual focal area and the corresponding effective focal spot- It can thus be readily seen that I have provided an X-ray tube in which the electron emissivity from a thermionic cathode is greater in one portion thereof than at other portions and the electron emission increases or decreases in accordance with variations in load impressed upon the X-ray tube. In varying the electron emissivity in accordance with variations in load the actual focal area and the effective focal spot are likewise varied therewith. Furthermore, in providing a cathode by which a line, oval or V- shaped focal area is produced, upon an anode theeffective useful life of the anode and X-ray tube is increased, due to the increased heat convection of these areas over the round spots of the prior art.

Although I have shown and described several embodiments of my invention I do not desire cations of the same may be made without departing from the spirit and scope of the appended claims.

What is claimed is: a

1. An X-ray tube comprising an evacuated envelope; an anode, a helically wound electron emitting cathode adapted to be electrically heated for producing a focal. area upon said anode and having the convolutions thereof in closer proximity to each other at one portion thereof to produce a greater electron emissivity for a given temperature at that portion than the remaining portions, an electrostatic shield surrounding said cathode and in conductive communication therewith, said shield having the plane of one of its sides angularly disposed relative to the plane of said cathode to produce a greater electrostatic field concentration at one portion than the remaining portions and cooperating with the portions of said cathode having a lesser electron emissivity to maintain the emissivity of the latter below that portion of the cathode having'a greater electron emissivity for all temperatures of said cathode, and to automatically vary the size of the focal area in accordance with variations in current impressed upon said tube. Q

2. An X-ray tube comprising an evacuated envelope, an anode provided with an active surface, a helically wound electron emitting cathode having theindividual convolutions thereof in closer proximity to each other at one portion of said cathode than at the remaining portions for increasing the temperature and electron emissivity of the portion having the convolutions in closer proximity to each other than over the remaining portions and adapted to be elecless than the actual focal area produced on said anode surface, an electrostatic shield surrounding said cathode and in conductive communication therewith and having the plane of its side immediately adjacent said anode extending in an angular direction to the vertical plane of said cathode to cause a greater electrostatic field concentration about a portion of the latter, and said cathode and said shield cooperating to automatically vary the size of the actual area and efiective focal spot in accordance with variations in current impressed upon said tube.

3. An X-ray tube comprising an evacuated enclosing envelope, an anode, and a helically wound electron emitting cathode adapted to be electrically heated disposed at substantially right angles to the longitudinal axis of said anode, said cathode having the individual convolutions of the helical winding in closer proximity to each other at one portion of said cathode than at the remaining portions thereof for increasing the temperature and electron emissivity of that portion above that of the remaining portions when supplied with a given amount of heating current, and an electrostatic shield surrounding said cathode and having the plane of one of its sides angularly disposed relative to the plane of said cathode for producing a greater electrostatic field concentration at one portion than the remaining portions thereof, said shield cooperating with said cathode to cause a greater electron emissivity at the portion of said cathode having the individual convolutions thereof in closer proximity to each other and with the remaining portions to cause an increase in electron emissivity at these portions upon an increase in heating current.

4. An X-ray tube comprising an evacuated enclosing envelope, an anode, a helically wound electron emitting cathode adapted to be electrically heated'and to project a focal area upon said anode, said cathode having the individual convolutions in closer proximity to each other at one portion thereof to produce a greater electron emissivity at that portion than over the remaining portions, an electrostatic shield surrounding said cathode and in conductive communication therewith and having one of its sides angularly disposed relative to the plane of said cathode to produce a maximum electrostatic field about that portion of the cathode having a lesser electron emissivity and a minimum electrostatic field about that portion of saidcathode having the greater electron emissivity, said shield and said cathode cooperating to increase the electron emissivity of the remaining lesser electron emitting portions of the latter in response to variations in heating current for varying the size of the produced focal area.

5. AnX-ray tube comprising an evacuated enclosing envelope, an anode, an electron emitting cathode having a substantially V-shaped configuration adapted to be electrically heated and project a focal area upon said anode varying in size with variations in load impressed upon said tube, said cathode being helically wound with the individual convolutions in closer proximity at the apex of the cathode than at the remaining portions for increasing the temperature and electron emissivity at the apex for a given amount of tube load, an electrostatic shield in conductive communication with said cathode and surrounding the same, said shield having the plane of its side immediately adjacent the anode extending in an angular direction to the vertical plane of said cathode and disposed a greater distance from the base of the cathode than at the remaining portions for concentrating a greater electrostatic held at the base, said cathode adaptedito increase in electron emissivity over the remaining portions and increase the focal area upon an increase in tube load.

6. An X-ray tube comprising an evacuated envelope, an anode, a helically wound electron emitting cathode adapted to be electrically heated and having the individual convolutions thereof in closer proximity at one portion thereof than at the remaining portions to produce a fixed'focal area having certain heat convection characteristics upon said anode for a given amount of electrical energy impressed upon said tube, and an electrostatic shield surrounding said cathode and in conductive communication therewith having the plane of one of its sides angularly disposed relative to the plane of said cathode and cooperating with the latter in producing a greater electron emissivity at the portion having the convolutions in closer proximity to each other, said shield and'said cathode being operable to cause a variation in the electron emissivity of theremaining portions of said cathode to vary the size and heat convection characteristics of the focal area in accordance with variations in electrical energy impressed upon said tube. 7

- ARTHUR MUTSCHELLER. 

